NHS Staff Accommodation for St Peter’s Hospital in Chertsey, Piled Rafts Design

Beta Scope

Beta Design Consultants comprised the following:

• Design of a piled raft system that can be supported on shallow piles. This system removes the need for a deep piling rig. This system shall replace the traditional system of ground beams on deep piles. This replaces the traditional system of ground beams on deep piles.
• Detailed construction drawings for 7 rafts including pile setting out and rebar drawings.

Background

Beta Design Consultants was commissioned to provide the geotechnical analysis and foundations design for the seven blocks of NHS Accommodation at St Peters Hospital in Chertsey. The scheme aims to provide 75 accommodation units in the first stage.

The 7 blocks foundation system would need to respond to the site conditions as laid out in the soil investigation, observe the project tight schedule and save on construction cost to stay within the budget.

BDC re-design of the substructure system lead to 3 weeks savings in the client programme from 14 weeks to 11 weeks.

Beta Solution

1. Design of Piled Raft System

The initial scheme that was proposed by others was to support each block on a system of ground beams that are supported by deep piles. BDC proposed an alternative system of piled rafts that offers the following advantages:

• Rafts are supported on shallow piles. These can be installed by a “mid-range” piling rig that takes piles to less than 10m deep. The rig would not need a piling mat so this would save a huge cost of the piling platform (usually an excavation of 500mm, compaction of soil that needs to be imported and compacted in layers, cost of nearly 30-40k per block, programme of at least 3 weeks per block).
• The piling programme has more certainty as larger number of small piles are required which are easier to install than deeper piles.
• The rafts require minimum excavation and shuttering compared to ground beams. The reduction in excavation and shuttering lead to huge savings in cost and programme. It is estimated that this has lead to savings of 20-25k per block and programme savings of 8-10 working days.
• The rafts rebar can be laid in a very simple manner. Raft reinforcement is easier to lay than ground beams caged rebars. This is also believed to have contributed to savings in budget and programme.

2. Detailed construction
BDC worked with the piling contractor and the groundworks team to prepare buildable designs for the piled rafts and for the rafts reinforcement.